Beta-adrenergic modulation of transient inward current in guinea-pig cardiac myocytes. Evidence for regulation of Ca2(+)-release from sarcoplasmic reticulum by a cyclic AMP dependent mechanism

Focal initiation of sustained and nonsustained ventricular tachycardia in a canine model of ischemic cardiomyopathy

INTRODUCTION

To define the role of focal and reentrant mechanisms underlying nonsustained (NSVT) and sustained ventricular tachycardia (SuVT) induced by programmed stimulation, 3-dimensional cardiac mapping was performed in 8 dogs with heart failure (HF) created by multiple intracoronary microsphere embolizations.

METHODS AND RESULTS

Continuous recording from 232 intramural sites throughout the left and right ventricles and the interventricular septum was performed during programmed stimulation in the absence and presence of isoproterenol (Iso, 0.1 μg/kg/min). Sinus beats and the last extrastimuli preceding induced VT conducted with total activation times (TA) of 51 ± 10 and 111 ± 8 milliseconds, respectively, that did not change during Iso infusion (47 ± 4 and 109 ± 5 milliseconds, P = NS). NSVT was induced in 75% of HF dogs; SuVT was induced in 38%. In all cases, initiation and maintenance of SuVT and NSVT arose by a focal mechanism. Compared to NSVT, SuVT had a shorter coupling interval (CI; 150 ± 7 vs 186 ± 16, P < 0.05) and a predilection for certain critical subendocardial initiation sites (that were initiation sites for only 29% of NSVT beats). After 21-30 beats, acceleration of SuVT by a focal mechanism to a CI less than 120 milliseconds led to functional conduction delay (TA increasing from 111 ± 3 to 137 ± 3 milliseconds, P < 0.0001), intramural reentry, and transition to ventricular fibrillation.

CONCLUSIONS

Thus, initiation of SuVT in a model of ischemic HF is due to a focal mechanism. However, subsequent acceleration of this focal mechanism can ultimately lead to functional conduction delay and development of intramural reentry.

Differences in Ca2+-management between the ventricle of two species of neotropical teleosts: the jeju, Hoplerythrinus unitaeniatus (Spix & Agassiz, 1829), and the acara, Geophagus brasiliensis (Quoy & Gaimard, 1824)

This study analyzed the physiological role of the cardiac sarcoplasmic reticulum (SR) of two neotropical teleosts, the jeju, Hoplerythrinus unitaeniatus (Erythrinidae), and the acara, Geophagus brasiliensis (Cichlidae). While the in vivo heart frequency (fH - bpm) of acara (79.6 ± 6.6) was higher than that of the jeju (50.3 ± 2.7), the opposite was observed for the ventricular inotropism (Fc - mN/mm²) at 12 bpm (acara = 28.66 ± 1.86 vs. jeju = 36.09 ± 1.67). A 5 min diastolic pause resulted in a strong potentiation of Fc (≅ 90%) of strips from jeju, which was completely abolished by ryanodine. Ryanodine also resulted in a ≅ 20% decrease in the Fc developed by strips from jeju at both subphysiological (12 bpm) and physiological (in vivo) frequencies. However, this effect of ryanodine reducing the Fc from jeju was completely compensated by adrenaline increments (10-9 and 10-6 M). In contrast, strips from acara were irresponsive to ryanodine, irrespective of the stimulation frequency, and increases in adrenaline concentration (to 10-9 and 10-6 M) further increased Fc. These results reinforce the hypothesis of the functionality of the SR as a common trait in neotropical ostariophysian (as jeju), while in acanthopterygians (as acara) it seems to be functional mainly in 'athletic' species.

Overexpression of beta 1 and beta 2 adrenergic receptors in rat atrial myocytes. Differential coupling to G protein-gated inward rectifier K(+) channels via G(s) and G(i)/o

G protein-activated inwardly rectifying K(+) (GIRK) channels, expressed in atrial myocytes, various neurons, and endocrine cells, represent the paradigmatic target of beta gamma subunits released from activated heterotrimeric G proteins. These channels contribute to physiological slowing of cardiac frequency and synaptic inhibition. They are activated by beta gamma dimers released upon stimulation of receptors coupled to pertussis toxin-sensitive G proteins (G(i/o)), whereas beta gamma released from G(s) do not converge on the channel subunits. This is in conflict with the finding that dimeric combinations of various beta and gamma subunits can activate GIRK channels with little specificity. In the present study, we have overexpressed the major subtypes of cardiac beta-adrenergic receptors (beta(1)-AR and beta(2)-AR) in atrial myocytes by transient transfection. Whereas in native cells beta-adrenergic stimulation with isoproterenol failed to induce measurable GIRK current, robust currents were recorded from myocytes overexpressing either beta(1)-AR or beta(2)-AR. Whereas the beta(2)-AR-induced current showed the same sensitivity to pertussis toxin as the current evoked by the endogenous G(i/o)-coupled muscarinic M(2) receptor, isoproterenol-activated currents were insensitive to pertussis toxin treatment in beta(1)-AR-overexpressing myocytes. In contrast to a recent publication (Leaney, J. L., Milligan, G., and Tinker, A. (2000) J. Biol. Chem. 275, 921-929), sizable GIRK currents could also be activated by isoproterenol when the signaling pathway was reconstituted by transient transfection in two different standard cell lines (Chinese hamster ovary and HEK293). These results demonstrate that specificity of receptor-G protein signaling can be disrupted by overexpression of receptors. Moreover, the alpha subunit of heterotrimeric G proteins does not confer specificity to G beta gamma-mediated signaling.

Changes of cardiac calcium homeostasis in spontaneously hypertensive rats

The aim of the present study was to assess the alterations in cardiac Ca2+ homeostasis induced by hypertension using electrically paced right ventricular strips from Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). 2 Basal contractile force was higher in SHR than in WKY. Similarly, the beta-adrenoceptor agonist isoprenaline (10 nM-10 microM) induced a concentration-dependent positive inotropic effect that was higher in SHR than in WKY, which was in turn inhibited by the beta-adrenoceptor antagonist propranolol (1 microM) in both strains. 3 Preincubation of strips with the L-type Ca2+ channel blockers, nifedipine (1 microM) or verapamil (10 microM), markedly inhibited the isoprenaline response, the inhibition being higher in SHR than in WKY. However, this inhibition was minor by the T-type Ca2+ channel blocker mibefradil (10 microM). 4 Bay K 8644 (10 nM-10 microM), a L-type Ca2+ channel activator induced a concentration-dependent positive inotropic effect, that was greater in SHR than WKY. 5 Nifedipine and verapamil (both 0.1 nM-10 microM) inhibited in a concentration-dependent way the inotropic effect induced by 0.3 microM isoprenaline or 1 microM Bay K 8644. The inhibition was higher in SHR than in WKY. Mibefradil (0.1 nM-10 microM) only clearly inhibited the isoprenaline and Bay K 8644 inotropic effects at 10 microM in both strains. 6 The inhibitor of the sarcoplasmic reticulum Ca2+ release, ryanodine (10 nM-10 microM), was a more effective depressor of isoprenaline-induced response in SHR than in WKY. 7 These results suggest that cardiac Ca2+ homeostasis in SHR ventricular strips is altered compared with those of WKY, showing an increased Ca2+ entry through L-type Ca2+ channels and release from sarcoplasmic reticulum; the participation of T-type Ca2+ channels are irrelevant in this tissue.

Activation of muscarinic K+ current by beta-adrenergic receptors in cultured atrial myocytes transfected with beta1 subunit of heterotrimeric G proteins

Muscarinic K+ channels (IK(ACh)) in native atrial myocytes are activated by betagamma subunits of pertussis toxin (Ptx)-sensitive heterotrimeric G proteins coupled to different receptors. betagamma subunits of Ptx-insensitive Gs, coupled to beta-adrenergic receptors, do not activate native IK(ACh). In atrial myocytes from adult rats transfected with rat brain beta1 subunit IK(ACh) can be activated by stimulation of beta-adrenergic receptors using isoprenaline. This effect is insensitive to Ptx. These findings demonstrate for the first time promiscuous (Ptx-insensitive) coupling of Gsbetagamma to GIRK channels in their native environment.

Withdrawal of acetylcholine elicits Ca2+-induced delayed afterdepolarizations in cat atrial myocytes

BACKGROUND

Recent experiments in atrial myocytes indicate that withdrawal of cholinergic agonist can directly increase Ca2+ influx via L-type Ca2+ current and stimulate Ca2+ uptake into the sarcoplasmic reticulum (SR), thereby increasing intracellular Ca2+. Overload of cellular Ca2+ within the SR can initiate various types of atrial dysrhythmias. The present study was designed to determine whether withdrawal of acetylcholine (ACh) can elicit Ca2+-induced delayed afterdepolarizations (DADs) in atrial myocytes.

METHODS AND RESULTS

A nystatin perforated-patch whole-cell method and fluorescence microscopy (indo 1) were used to measure electrical activities and intracellular free Ca2+ ([Ca2+]i), respectively. Withdrawal of ACh (1 micromol/L) increased action potential duration, shifted plateau voltage toward positive, and generated DADs that initiated spontaneous action potentials. Voltage-clamp analysis revealed that withdrawal of ACh elicited a rebound stimulation of L-type Ca2+ current (I(Ca,L)) (+45%) and Na/Ca exchange current (I(NaCa)) (+16%) and the appearance of transient inward current (I(ti)) and spontaneous [Ca2+]i transients. Each of these changes induced by withdrawal of ACh was abolished by Rp-cAMPs (50 to 100 micromol/L) or H-89 (2 micromol/L), inhibitors of cAMP-dependent protein kinase A. Ryanodine (1 micromol/L) abolished I(NaCa) and the appearance of I(ti) without decreasing the rebound stimulation of I(Ca,L) elicited by withdrawal of ACh.

CONCLUSIONS

Withdrawal of ACh can elicit cAMP-mediated stimulation of Ca2+ influx via I(Ca,L) and uptake of SR Ca2+. As a result, cellular Ca2+ overload causes enhanced SR Ca2+ release and the initiation of DADs. These mechanisms may generate triggered and/or spontaneous atrial depolarizations elicited by withdrawal of vagal nerve activity.

Contribution of Na(+)-Ca2+ exchange to stimulation of transient inward current by isoproterenol in rabbit cardiac Purkinje fibers

Cellular mechanisms underlying beta-adrenergic stimulation of the arrhythmogenic transient inward current (TI) were investigated by using a two-microelectrode voltage-clamp technique in rabbit cardiac Purkinje fibers. TI induced by elevating [Ca2+]o to 30 mmol/L and substituting [Na+]o with N-methyl-D-glucamine (NMG) chloride had a distinct reversal potential (EREV) of -25 mV, suggesting that Na(+)-Ca2+ exchange was not the charge carrier for TI. In the absence of [Na+]o, isoproterenol (ISO, 0.01 to 5.0 mumol/L) had no effect on either inward or outward TI or on the current-voltage relation of TI. However, ISO (0.1 mumol/L) significantly increased both inward and outward TIs without affecting the EREV of TI, if [Na+]o was present. Pretreatment with propranolol (0.2 mumol/L) or atenolol (0.2 mumol/L) abolished the stimulatory effects of ISO. Addition of propranolol (0.2 to 0.5 mumol/L) after the effects of ISO had developed caused only partial reversal of TI stimulation. This indicates persistence of stimulatory effects downstream from the initial agonist-receptor interaction. Forskolin (1 mumol/L), a direct adenylate cyclase activator, also strongly increased both inward and outward TI in the presence of [Na+]o. These effects also were abolished when [Na+]o was substituted by NMG. Inward and outward TIs enhanced by either ISO or forskolin were reversed by two putative Na(+)-Ca2+ exchange blockers, dodecylamine (20 mumol/L) and quinacrine (20 mumol/L). These results suggest that beta-adrenergic stimulation of TI is mediated by the Na(+)-Ca2+ exchange; stimulation likely involves phosphorylation of the exchanger or some factor that modulates exchanger activity.

Modulation of Ca2+ release in cultured neonatal rat cardiac myocytes. Insight from subcellular release patterns revealed by confocal microscopy

It is well established that in heart muscle the influx of Ca2+ through Ca2+ channels during the action potential is the main trigger for Ca2+ release from the sarcoplasmic reticulum (SR), but intact cardiac tissue and single myocytes are also known to exhibit spontaneous Ca2+ release from the SR under a variety of circumstances. Although conditions favoring spontaneous activity have been examined extensively, mechanisms modulating or regulating spontaneous as well as triggered Ca2+ release are still largely unknown. Using the high spatial and temporal resolution of laser-scanning confocal microscopy, we investigated subcellular aspects of spontaneous and triggered Ca2+ release in isolated rat neonatal myocytes loaded with the Ca(2+)-sensitive fluorescent dye fluo 3. Three distinct patterns of spontaneous Ca2+ release were identified: (1) a homogeneous Ca2+ release, presumably corresponding to Ca2+ release during a spontaneous action potential, (2) a focal or spatially restricted Ca2+ release with no or only limited subcellular propagation, and (3) a Ca2+ release propagating as a wave throughout the entire cell. Pharmacologic tools that interfere with the SR revealed that all release types were critically dependent on the Ca2+ release and uptake function of the SR. From our results we conclude that the probability, extent, and pattern of Ca2+ release are modulated on the subcellular level. The observed spectrum of release patterns can be explained by a space- and time-dependent variability in the positive feedback of the Ca(2+)-induced Ca(2+)-release mechanism within an individual myocyte. Presumably, this variability depends on the existence of subcellular functional elements of the SR. The actual degree of positive feedback may be modulated locally by the Ca(2+)-loading state of each SR element.

Hypoxia enhances isoproterenol-induced increase in heart interstitial adenosine, depressing beta-adrenergic contractile responses

Endogenous interstitial adenosine may protect the hypoxic heart by attenuating beta-adrenergic-induced contractile and metabolic responses, thereby reducing energy utilization. Constant-flow perfused rat hearts were used to study: 1) the effect of hypoxia on isoproterenol (ISO)-induced increase in interstitial adenosine, as estimated with epicardial surface transudates, and 2) the role of endogenous adenosine in hypoxic depression of ISO-induced contractile responses. ISO (1 nM for 10 minutes) in the normoxic heart increased transudate adenosine 114% from a pre-ISO normoxic value of 343 pmol/ml. ISO administered to the hypoxic heart increased transudate adenosine 357% from a pre-ISO hypoxic value of 797 pmol/ml. The absolute magnitude of the ISO-induced increase in transudate adenosine was 625% greater during hypoxia than during normoxia. This was associated with a reduction in the ISO-induced contractile response during hypoxia. In other experiments, with normoxia ISO (10 nM for 10 seconds) increased developed left ventricular pressure by 140 mm Hg, and the maximum rates of left ventricular pressure development and relaxation by 5,860 and 2,771 mm Hg/sec, respectively, above control values of 90 mm Hg, 2,250 mm Hg/sec, and 1,875 mm Hg/sec. Hypoxia reduced the three ISO-induced contractile responses by 50%, 56%, and 36%. However, 1,3-dipropyl-8-cyclopentylxanthine (5 x 10(-7) M), an adenosine A1-receptor antagonist, added to the hypoxic hearts resulted in only a 31%, 39%, and 9% reduction in the ISO-induced responses in developed left ventricular pressure and the maximum rates of left ventricular pressure development and relaxation, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium transients caused by calcium entry are influenced by the sarcoplasmic reticulum in guinea-pig atrial myocytes

1. Single atrial myocytes obtained by enzyme perfusion from hearts of adult guinea-pigs were investigated using whole-cell voltage clamp and Indo-1 micro-fluorometry. 2. In myocytes loaded with a solution containing citrate as a low-affinity, non-saturable Ca2+ chelator, two types of [Ca2+]i transients could be recorded during repetitive activation of L-type Ca2+ current. Both large and small [Ca2+]i transients occurred; large transients reached peak values of about 1 microM, and small transients were about 100 nM or less in amplitude. 3. In the case of the large transients, peak [Ca2+]i was usually reached with a variable delay after repolarization from a voltage step that activated calcium current (ICa). For the small transients the rise in [Ca2+]i paralleled ICa. Upon repolarization [Ca2+]i started to decay. 4. The small transients reflect entry of Ca2+ through Ca2+ channels (entry transients), whereas the large transients are due to entry and release from the sarcoplasmic reticulum (release transients). 5. The entry transients displayed a positive staircase pattern during trains of depolarizing voltage steps despite constant or even decreasing amplitude of ICa. The steepness of the staircase was increased by elevation of [Ca2+]o. Entry transients were always smallest immediately after a release transient. 6. After functional removal of the sarcoplasmic reticulum by caffeine (1-5 mM) the staircase pattern of the transients reflecting Ca2+ entry was abolished. 7. It is concluded that the staircase pattern is due to rapid uptake by the sarcoplasmic reticulum of Ca2+ entering the cell, resulting in an attenuation of the signal. The attenuation is strongest shortly after a release signal, when the rate of sequestration of Ca2+ by the SR should be highest. 8. Evidence is provided that a compartment of the SR is involved in attenuation of the entry transients. This compartment has been identified recently as a peripheral release compartment.

Effects of intracellular Ca2+ chelating compounds on inward currents caused by Ca2+ release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ release from the sarcoplasmic reticulum (SR) of mammalian cardiac myocytes occurring either due to activation by a depolarization or the resulting transmembrane Ca2+ current (ICa), or spontaneously due to Ca2+ overload has been shown to cause inward current(s) at negative membrane potentials. In this study, the effects of different intracellular Ca2+ chelating compounds on ICa-evoked or spontaneous Ca(2+)-release-dependent inward currents were examined in dialysed atrial myocytes from hearts of adult guinea-pigs by means of whole-cell voltage-clamp. As compared to dialysis with solutions containing only a low concentration of a high affinity ethylene glycol-bis(beta-aminoethylether) N,N,N',N'-tetraacetic acid (EGTA) like chelator (50-200 microM), inward membrane currents (at -50 mV) due to evoked Ca2+ release, spontaneous Ca2+ release or Ca2+ overload following long-lasting depolarizations to very positive membrane potentials are prolonged if tne dialysing fluid contains a high concentration of a low affinity Ca2+ chelating compound such as citrate or free adenosine 5'-triphosphate (ATP). Without such a non-saturable Ca2+ chelator in the dialysing fluid, Ca(2+)-release-dependent inward currents are often oscillatory and show an irregular amplitude. With a low affinity chelator in a non-saturable concentration, discrete inward currents with constant properties can be recorded. We conclude that the variability in Ca(2+)-release-dependent inward current seen in single cells arises from spatial inhomogeneities of intracellular Ca2+ concentration ([Ca2+]i) due to localized saturation of endogenous and exogenous high affinity Ca2+ buffers (e.g.). This can be avoided experimentally by addition of a non-saturable buffer to the intracellular solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of Ca2(+)-release-dependent inward current reveals two distinct components of Ca2+ release from sarcoplasmic reticulum in guinea-pig atrial myocytes

Ca2+ current (L-type) and inward current caused by Ca2+ release from the sarcoplasmic reticulum and carried by electrogenic Na+/Ca2+ exchange have been measured in cultured atrial myocytes from hearts of adult guinea-pigs using whole-cell voltage clamp techniques. The pipette solution, used for internal dialysis of the cells, contained a high concentration, 60 mM or 25 mM, of citrate as a non-saturable low-affinity Ca2(+)-chelating compound. It has been shown previously that Ca2(+)-release-dependent inward current under these conditions is carried by electrogenic Na+/Ca2+ exchange. Furthermore, Ca2(+)-release-dependent inward current (the release signal) can be completely separated from triggering Ca2+ current if brief depolarizations for activating ICa are used. In the majority of cells that did not produce spontaneous Ca2+ release, conditions could be found that caused the release signal to be split into two components: an early component of variable amplitude and a late component of rather constant amplitude. The delay of the late component with regard to triggering ICa was inversely related to the amplitude of the first one. Below a certain amplitude of the first component, the second one failed to be elicited. This suggests the second component to be triggered by the first one. Weakly Ca2(+)-buffered cells produced spontaneous Ca2+ release, resulting in irregular "transient inward currents" at constant membrane-holding potential. Synchronization by trains of step depolarizations unmasked two components also in the spontaneous release signals. In none of the cells studied was any indication of more than two components of the release signal detected. The results are discussed in terms of two distinct compartments of sarcoplasmic reticulum with different properties of Ca2+ release.

Simultaneous recording of Indo-1 fluorescence and Na+/Ca2+ exchange current reveals two components of Ca2(+)-release from sarcoplasmic reticulum of cardiac atrial myocytes

Simultaneous measurements of intracellular Ca2(+)-concentration ([Ca2+]i) using Indo-1 and the current generated by electrogenic Na+/Ca2(+)-exchange (INaCa) have been performed on atrial myocytes from hearts of adult guinea-pigs. Whereas the fluorescence-measurements provide information on global [Ca2+]i, InaCa which is a linear function of Ca2(+)-concentration, indicates subsarcolemmal [Ca2+]. Under conditions in which intracellular Ca2(+)-transients due to Ca2(+)-release from the sarcoplasmic reticulum (SR) have been artificially slowed, a deviation between the two different Ca2(+)-signals can be found. During onset of release signals Ca2(+)-concentration seen by the membrane is higher than global [Ca2+]i. Our results provide evidence that in atrial myocytes, lacking a T-system, Ca2(+)-induced Ca2(+)-release occurs first from a subsarcolemmal compartment of the SR. The resulting Ca2(+)-transient serves to trigger Ca2(+)-release from deeper SR-compartments.

Large-conductance ion channel measured by whole-cell voltage clamp in single cardiac cells: modulation by beta-adrenergic stimulation and inhibition by octanol

Membrane currents in single cardiac myocytes from adult guinea pigs were studied by means of the patch-clamp technique (whole-cell mode). During spontaneous or caffeine-induced Ca2+ release from the sarcoplasmic reticulum openings of a novel ion channel with large unitary conductance (280 pS) can be recorded. The density of these channels and/or its open-state probability are unusually low. On average in the whole-cell mode simultaneous maximum superposition of only four channels is observed. Opening events of this channel require an intracellular Ca2+ transient. Activation by [Ca2+]i, however, seems to be indirect; maximum opening activity occurs with a delay of several hundred milliseconds after peak [Ca2+]i. Single-channel activity can be enhanced by a cyclic AMP dependent process via beta-adrenergic stimulation of a cell. This can also be mimicked by caffeine, most likely via inhibition of phosphodiesterase. Octanol, an inhibitor of gap-junctional coupling in a variety of tissues, causes a concentration-dependent and reversible decrease in single-channel activity. Unitary conductance is not affected by octanol. The low density of these channels in cardiac membranes and their poor selectivity render any role in normal cardiac electrical activity unlikely. A possible relation of the channel to cardiac gap junctions is discussed.